Electronically scanned array antennas are commonly used in air, space and ground communication systems. These array antennas comprise multiple antenna elements whose radiation patterns are constructively combined to form antenna beams. By controlling the phase and/or amplitude of the signal fed to the individual antenna elements, the generated antenna beams are electronically shaped and scanned in a desired direction. Because the antenna beam is controlled electronically, these array antennas require minimal mechanical structure and moving parts, and are preferred for use on satellite communication systems.
The radiation pattern of an array antenna is the product of the array pattern and the radiation pattern of the individual antenna elements in the array. Desired radiation pattern characteristics, such as high directivity, low side lobes, and the absence of grating lobes, are sought after by modifying the array pattern and/or the individual antenna elements. For example, the directivity of an array antenna can be increased by increasing the aperture size of the array antenna. If a sparse array is used to obtain the larger aperture size, however, grating lobes can be generated in the radiation pattern thereby reducing the directivity of the array antenna.
Another desirable feature of array antennas is the ability to operate in multiple frequency bands and/or transmit multiple signals. For example, transmission array antennas are often required to transmit two different signals. Conventional array antennas often meet this requirement by using antenna elements designed to radiate both signals. However, when both signals pass through a non-linear circuit within the array antenna, intermodulation products from third order mixing can cause spurious signals to appear in or near the pass-bands associated with the intended transmission signals.
Accordingly, a need exists for array antenna designs that generate desirable radiation patterns. The array antenna designs should be robust enough to handle multiple signals in multiple frequency bands. The array antenna designs should also allow lightweight, thin-profile implementations having relatively low costs.